Liquid crystal polymers are a family of materials that exhibit a highly ordered structure in the melt, solution, and solid states. They can be broadly classified into two types; lyotropic, having liquid crystal properties in the solution state, and thermotropic, having liquid crystal properties in the melted state.
Most liquid crystal polymers exhibit excellent physical properties such as high strength, good heat resistance, low coefficient of thermal expansion, good electrical insulation characteristics, low moisture absorption, and are good barriers to gas flow. Such properties make them useful in a broad range of applications in the form of fibers, injection molded articles, and, in sheet form, as electronic materials for printed circuit boards, packaging, and the like.
Many of the physical properties of liquid crystal polymers are very sensitive to the direction of orientation of the liquid crystal regions in the polymer. The ordered structure of the liquid crystal polymer is easily oriented by shear forces occurring during processing and highly aligned liquid crystal chains can be developed that are retained in the solid state, and result in highly anisotropic properties. This can be highly desirable for certain products, for example, in filaments, fibers, yarns, and the like. Anisotropic properties are often not desirable, however, in products having planar forms, such as tape, films, sheet, and the like.
A number of methods are used to produce liquid crystal polymer materials in planar forms that have more balanced, less anisotropic properties. These include the use of multilayer flat extrusion dies which are fashioned such that they extrude overlapping layers at intersecting angles, use of static mixer-agitators at the die inlets, and the like. More recently, dies having rotating or counter-rotating surfaces have become known in the art and successfully used. These extrusion techniques, used separately or in combination with other methods known in the art, such as film blowing, can produce liquid crystal polymer film and sheet that are multiaxially oriented, that is, oriented in more than one direction, and have more balanced physical properties.
A characteristic of these methods is that locally, for example, at the surfaces of the sheet or film, the molecules are oriented in the planar x-y directions by shear imparted at the extrusion surfaces. In the z-direction, i.e., throughout the thickness, the x-y orientation of the molecules will change progressively from the orientation at one surface to the orientation at the other surface of the planar form. A drawback to these methods is that when attempting to make very thin multiaxially oriented films, e.g., films having a thickness of 25 micrometers or less, the forces imparted in the orientation transition region of the liquid crystal polymer by the extrusion surfaces are exerted in increasingly opposing directions as the distance between the extrusion surfaces diminishes and the formation of pinholes, tears, and other imperfections in the film takes place.
Additionally, and particulary in the case of melt-processed thermotropic liquid crystal polymers which have very high processing viscosity, it is difficult to obtain films with uniform surface smoothness and thickness by the processes described above. Problems such as film curling, streaking, and tear sensitivity have been associated with surface roughness and irregularities or nonuniform thickness in the films.